This invention relates to an electrically programmable nonvolatile semiconductor memory device which can be easily subjected to an electrical characteristic test with high reliability.
In recent times, a so-called UV-EPROM has come to be the most popular nonvolatile memory or programmable ROM (hereinafter referred to as a PROM). Data can be electrically programmed in the UV-EPROM and erased therefrom by use of ultraviolet radiation. It is well known that data is, in fact, rarely programmed in or erased from such a UV-EPROM very many times; mostly, it is programmed only once. However, since a memory cell of the UV-EPROM must be irradiated with ultraviolet rays in order to erase data, the memory chip is sealed into a package having a glass window for enabling the transmitting of ultraviolet rays and, hence, this package is expensive. For this reason, if data is programmed only once and will not need to be erased in the future, the memory chip is simply sealed into an inexpensive plastic package not having a window for the transmitting of ultraviolet rays. Such a UV-EPROM is called one-time PROM. In this type of one-time PROM, data cannot be erased after it has been programmed in.
After the memory chip is sealed into the package, a semiconductor memory is subjected to tests with predetermined data programmed therein, so as to determine whether various electrical characteristics of the memory chip (e.g., data readout time or rate and current consumption) satisfy the respective rated values. Memory chips not satisfying the rated values are rejected, and only those memory chips satisfying the rated values are shipped as products, after the test data is erased. However, data programmed in the one-time PROM sealed into a plastic package cannot be erased, and hence one-time PROMs cannot be subjected to the tests described above. Thus, in order to allow a user to program given data, a memory chip must be shipped in which no data is programmed.
Therefore, in order to perform the above tests, data is programmed in a memory chip before it is sealed into the package, i.e., when the memory chips are still in the form of a wafer, and then data is read out to check the readout rate. Thereafter, data is erased and memory chips satisfying the predetermined read rate are sealed into packages. However, according to this method, the readout rate of the memory cell sealed into the package cannot be directly checked because the memory chips are subjected to the tests while they remain in the form of a wafer. That is, a slight difference in the readout rate occurs before and after the memory chip is sealed into the package. For this reason the critical value of the readout rate is set slightly higher than the predetermined value when the test is performed for the memory chips still in wafer form. Any memory chips having a readout rate lower than the critical value is eliminated. As a result, the product yield drops, thereby increasing the manufacturing cost. In addition, since the memory chip may be damaged when it is sealed into the package, some sealed memory cells may become defective. If a user programs data in such a memory chip, the data readout rate may be greatly decreased, or erroneous data may be read out. Consequently, it is desirable to test the one-time PROMs after they have been sealed into the package.